1. Field of the Invention
The present invention relates to a light emitting display apparatus and a driving method thereof, and more particularly, to a light emitting display apparatus, capable of reducing pixel deterioration overcurrent by preventing over current, and a driving method thereof.
2. Discussion of Related Technology
Various flat panel displays, which are relatively lighter and smaller than a cathode ray tube (CRT) display, have recently been developed. Among the flat panel displays, a light emitting display having an excellent viewing angle, fast speed response, high definition, a thin structure, and the like is desired.
Such desirable properties can be found in a light emitting display comprising an organic light emitting display (OLED) which uses an organic light emitting element, and an inorganic light emitting display which uses an inorganic light emitting element. The organic light emitting element is also called an organic light emitting diode (OLED). An OLED comprises an organic light emitting layer disposed between an anode electrode and a cathode electrode for emitting light. The inorganic light emitting element is also called a light emitting diode (LED). A LED comprises a light emitting diode formed of an inorganic material, for example, a semiconductor with p-n junction different from the organic light emitting diode.
FIG. 1 illustrates a circuit diagram of a pixel employed in a typical light emitting display apparatus. Referring to FIG. 1, a pixel comprises a pixel circuit, an OLED, a first transistor M1, a second transistor M2, and a capacitor Cst. Each of the first and second transistors is implemented by a PMOS transistor and is composed of a source, a drain and a gate. Since the source and the drain are physically identical, they can be just called a first electrode and a second electrode or vice versa. The capacitor Cst comprises a first terminal and a second terminal.
The first transistor M1 has the source connected to a first power line ELVdd, a drain connected to the OLED and a gate connected to a first node N1. Accordingly, a current flows from the source to the drain in response to a voltage at the first node N1.
The second transistor M2 has the source connected to a data line Dm, a drain connected to the first node N1 and a gate connected to a scan line Sn, so that the second transistor M2 selectively transfers a data signal flowing through the data line Dm to the first node N1 depending on a scan signal transmitted via the scan line Sn.
The capacitor Cst is comprised of a first terminal connected to the first power line ELVdd and a second terminal connected to the first node N1, thereby maintaining a voltage between the gate and the source of the first transistor M1 for a predetermined time. The voltage stored in the capacitor Cst is computed by the following equation:Vgs=ELVdd−Vdata   Equation (1)
Here, Vgs is a gate-to-source voltage of the first transistor M1, ELVdd is a voltage of the first power line and Vdata is a voltage of the data signal.
Further, a current flowing across the OLED is computed by the following equation:
                              I          OLED                =                                            β              2                        ⁢                                          (                                  Vgs                  -                  Vth                                )                            2                                =                                    β              2                        ⁢                                          (                                  ELVdd                  -                  Vdata                  -                  Vth                                )                            2                                                          Equation        ⁢                                  ⁢                  (          2          )                    
Here, IOLED is a current flowing through the OLED, Vgs is a gate-to-source voltage of the first transistor M1, ELVdd is a voltage of the first power line, Vdata is a voltage of the data signal and Vth is a threshold voltage of the first transistor M1.
Accordingly, a light emitting device comprising the pixel as illustrated in FIG. 1 emits light by making current flow through the OLED.
However, if overcurrent, i.e., current exceeding the maximum rated current for the device, flows across the OLED for a long time, it causes damage to the OLED, so that the element continues to deteriorate and finally cannot emit light.